Method and apparatus for directional radio communication

ABSTRACT

A method of directional radio communication between a first station ( 6 ) and a second station, comprising the steps of defining at the first station ( 6 ) a plurality of beam directions (b 1 -b 8 ) for transmitting signals to said second station, each of said beam directions being selectable. A plurality of beam directions are selected at said first station in which a signal is to be transmitted form said first station to said second station. Said signal i transmitted in said plurality of beam directions, whereby the power level of the signal transmitted in each of said selected plurality of beam directions is individually selectable.

The present invention relates to a method and apparatus for directionalradio communication in which signals between a first station and asecond station may be transmitted only in certain directions. Inparticular, but not exclusively, the present invention is applicable tocellular communication networks using space division multiple access(SDMA).

With currently implemented cellular communication networks, a basetransceiver station (BTS) is provided which transmits signals intendedfor a given mobile station (MS), which may be a mobile telephone,throughout a cell or cell sector served by that base transceiverstation. However, space division multiple access (SDMA) systems have nowbeen proposed. In a space division multiple access system, the basetransceiver station will not transmit signals intended for a givenmobile station throughout the cell or cell sector but will only transmitthe signal in the beam direction from which a signal from the mobilestation is received. SDMA systems may also permit the base transceiverstation to determine the direction from which signals from the mobilestation are received.

SDMA systems may allow a number of advantages over existing systems tobe achieved. In particular, as the beam which is transmitted by the BTSmay only be transmitted in a particular direction and accordingly may berelatively narrow, the power of the transceiver can be concentrated intothat narrow beam. Likewise, the signal transmitted to the BTS by, forexample, a MS will be received by the BTS only in a limited number ofbeam directions. It is believed that this results in a better signal tonoise ratio with both the signals transmitted from the base transceiverstation and the signals received by the base transceiver station.Additionally, as a result of the directionality of the base transceiverstation, an improvement in the signal to interference ratio of thesignal received by the base transceiver station can be achieved.Furthermore, in the transmitting direction, the directionality of theBTS allows energy to be concentrated into a narrow beam so that thesignal transmitted by the BTS can reach far away located mobile stationswith lower power levels than required by a conventional BTS. This mayallow mobile stations to operate successfully at greater distances fromthe base transceiver station which in turn means that the size of eachcell or cell sector of the cellular network can be increased. As aconsequence of the larger cell size, the number of base stations whichare required can also be reduced leading to lower network costs. SDMAsystems generally require a number of antenna elements in order toachieve the required plurality of different beam directions in whichsignals can be transmitted and received. The provision of a plurality ofantenna elements increases the sensitivity of the BTS to receivedsignals. This means that larger cell sizes do not adversely affect thereception of signals by the BTS from mobile stations.

SDMA systems may also increase the capacity of the system, that is thenumber of mobile stations which can be simultaneously supported by thesystem is increased. This is due to the directional nature of thecommunication which means that the BTS will pick up less interferencefrom mobile stations in other cells using the same frequency. The BTSwill generate less interference to other mobile stations in other cellsusing the same frequency when communicating with a given MS in theassociated cell.

Ultimately, it is believed that SDMA systems will allow the samefrequency to be used simultaneously to transmit to two or even moredifferent mobile stations which are arranged at different locationswithin the same cell. This can lead to a significant increase in theamount of traffic which can be carried by cellular networks.

SDMA systems can be implemented in analogue and digital cellularnetworks and may be incorporated in the various existing standards suchas GSM, DCS 1800, TACS, AMPS and NMT as well as he proposed nextgeneration standards such as, for example, UMTS (Universal MobileTelecommunications System). SDMA systems can also be used in conjunctionwith other existing multiple access techniques such as time divisionmultiple access (TDMA), code division multiple access (CDMA) andfrequency division multiple access (FDMA) techniques.

One problem with SDMA systems is that the direction in which signalsshould be transmitted to a mobile station needs to be determined. Incertain circumstances, a relatively narrow beam will be used to send asignal from a base transceiver station to a mobile station. Therefore,the direction of that mobile station needs to be assessed reasonablyaccurately. As is known, a signal from a mobile station will generallyfollow several paths to the ETS. Those plurality of paths are generallyreferred to as multipaths. A given signal which is transmitted by themobile station may then be received by the base transceiver station frommore than one direction due to these multipath effects.

In general, the decision as to the beam direction which is to be used bythe BTS in order to transmit a signal to a mobile station is based oninformation corresponding to the data burst previously received by theBTS from the given MS. As the decision is based on information receivedcorresponding to only one burst, problems may occur if, for example, thedata burst transmitted by the mobile station is superimposed with stronginterference.

An additional problem is that the direction in which a signal is to betransmitted by the BTS to the mobile station is determined on the basisof the signals received by the BTS from the mobile station. However, thefrequencies of the signals transmitted from the mobile station to theBTS are different from the frequencies used for the signals transmittedby the BTS to the mobile station. The difference in the frequencies usedin the uplink and downlink signals means that the behaviour of thechannel in the uplink direction may be different from the behaviour ofthe channel in the downlink direction. Thus the optimum directiondetermined for the uplink signals will not always be the optimumdirection for the downlink signals. In other words, the statisticalbehaviour of the channel in the up-link and down-link directions aredifferent. This also means that it is not possible to have a fast andeffective (burst-by-burst basis) power control as there is in general nofast (burst-by-burst) feedback from the MS.

It has been proposed by the present inventor that a signal from a basetransceiver station to a mobile station be sent in two adjacent beamdirections. This means that the base station generates two or moreseparate beams. When those beams are adjacent one another, they shouldoverlap. By allowing the beams to overlap the whole cell or cell sectorcan be covered. However, due to differences in the effective path lengthtravelled by the signals to a beam former of the base transceiverstation, adjacent beams may have an effective phase differencetherebetween. Depending on the value of this phase difference, a nullregion may occur in the overlapping region of two adjacent beams. Anymobile station in that null region would be unable to receive signalsfrom the base transceiver station. Another problem arises when more thanone beam direction is selected. If the power of the beams is set to beequal, this can undesirably give rise to increased interference.

It is therefore an aim of certain embodiments of the present inventionto address some of the problems mentioned hereinbefore.

According to a first aspect of the present invention, there is provideda method of directional radio communication between a first station anda second station, said method comprising the steps of defining at thefirst station a plurality of beam directions for transmitting signals tosaid second station, each of said beam directions being selectable;selecting a plurality of beam directions at said first station in whicha signal is to be transmitted from said first station to said secondstation; consecutively transmitting said signal in said plurality ofbeam directions, whereby the power level of the signal transmitted ineach of said selected plurality of beam directions is individuallyselectable.

For the purpose of this document, the term signal should be broadlyinterpreted. For example, a burst of data in a GSM system may constitute“a signal”. Alternatively, a plurality of bursts of data in a GSM systemmay constitute “a signal”.

It has been recognized that better results can be achieved, where two ormore beams are selected if the power of each of the selected beams isindividually selectable.

By also altering the power of each beam, a more flexible shaping of thebeam pattern can be obtained thus reducing the possible interference tonon-desired stations. This in turn means that it might be possible toimprove the system capacity. Thus, if the power of each beam isindividually selectable, it is possible to flexibly alter the shape ofthe beam pattern.

Preferably, first and second sequential signals are to be transmitted tosaid second station, said method further comprising the steps of:

altering the phase of the first and second signals to be transmitted inat least one of said selected beam directions;

whereby the phase of the first signal transmitted in at least two of thebeam directions differs, the phase of the second signal transmitted insaid at least two beam directions differs and the phase difference ofthe first signal transmitted in said at least two beam directions isdifferent from the phase difference of said second signal transmitted insaid at least two beam directions.

By ensuring that the phase difference of the first signal transmitted inthe two beam directions is different from the phase difference of thesecond signal transmitted in the two beam directions, the problemscaused by null regions can be considerably reduced. In particular, evenif a null region occurs for one signal, it is unlikely to be present forthe second signal in that the phase difference between adjacent beams ischanged for the next signal. It should be appreciated that the phase ofthe signals transmitted in one beam direction may be unaltered but thephase of the signals in another beam direction may be altered.Alternatively, the phase of the signals may be altered for all of theselected beam directions.

Preferably, at least two of the beam directions are adjacent. It ispreferred that the first station be arranged to transmit a multiplicityof consecutive signals to the second station and that the phase of eachsignal be altered such that the phase difference between eachconsecutive signal transmitted in at least two of the beam directions isdifferent for consecutive signals. Thus, when the response of the firststation is averaged over time, the probability that null regions occurcan be reduced.

Preferably, the phase of the consecutive signals is randomly altered.The random alteration of the phase is able to create a spatialmodulation of the resulting beam pattern, particularly when consideredover a relatively large number of consecutive signals. However, it isalso possible that the phase of the consecutive signals can be alteredin accordance with a predetermined pattern. It is preferred that thispredetermined pattern allows the desired spatial modulation of the beampattern to be achieved.

Preferably, method also includes the steps of receiving at said firststation a plurality of signals from said second station, said signalsbeing receivable from a plurality of beam directions; determining for atleast one of said beam directions a value of a parameter of at least onesignal received from said second station in said at least one beamdirection; looking up in a look-up table a power value corresponding tothe determined value; and transmitting a signal to the second station insaid at least one beam direction, the power of the signal transmitted insaid at least one beam direction being determined by the power valuelooked up in the look-up table. Preferably the received signals areconsecutive

According to a second aspect of the present invention, there is provideda method of directional radio communication between a first and a secondstation, said method comprising the steps of receiving at said firststation a plurality of consecutive signals from said second station,said signals being receivable from a plurality of beam directions;determining for at least one of said beam directions a value of aparameter of at least one signal received from said second station insaid at least one beam direction; looking up in a look-up table a powervalue corresponding to the determined value; and transmitting a secondsignal to said second station in said at least one beam direction, thepower of the signal in said at least one beam direction being determinedby the power value looked up in the look-up table.

The use of a look-up table is particularly advantageous in that itprovides a simple way of determining the power level of a signal to betransmitted to the second station.

Preferably, a mean value of the parameter for a plurality of signals isdetermined and the power value corresponding to the determined meanvalue is looked up in a look-up table. This parameter may be the energyof the signals. Alternatively, the parameter can be one or more of thefollowing parameters: instantaneous energy of a signal; type of radioenvironment; or distance between the first and second stations.

There are advantages in being able to determine the power of a signal tobe transmitted to the second station based on an average of a number ofpreceding signals received from that second station. This is becausealthough for a single signal the behaviour of a channel between thefirst station and the second station is not the same as a channelbetween the second station and the first station, on average, thebehaviour of the channels in both directions will be similar. By takinginto account the signals received over a period of time, it can beassumed that the channel between the first station and the secondstation will on average be similar to that between the second stationand the first station. It will be noted that this method of determiningthe power will generally be effective on average and not necessarilyeffective for every single signal.

The mean energy determined in said determining step may be quantized andthe quantized mean energy be associated by the look-up table with acorresponding power value. This makes the look-up table easier toachieve in practice.

The power value represents the power of the signal to be transmitted inthe beam direction or alternatively may represent a control value forcontrolling the setting of a power level for a signal in a given beamdirection. For example, if an amplifier were present, the control valuecould be used to control the amplifier to provide the required signalamplification.

The values in the look-up table may be altered in accordance with aparameter of the first station and/or the second station. Alternatively,the values of the look-up table may be fixed and not be alterable. Wherethe values of the look-up table are altered, they may be altered inaccordance with the power measuring reports received from the secondstation. The values for the look-up table are preferably determinedbased on one or more of the following:

transmission power used by said second station;

distance between said first and second station;

the mean energy of the signals received from the second station in agiven beam direction;

distance between the first and second station;

the radio environment;

the validity of a known attenuation law in a channel defined between thefirst and second stations.

The number of signals used to calculate the mean value may be varieddepending for example on the degree of correlation between the channelbetween the first station and the second station and the channel betweenthe second station and the first station. The number of signals used tocalculate the mean value may be dependent on the signal quality reportsreceived from the second station.

The energy of each of the signals received in the given beam directionis preferably determined from the channel impulse response. Thiscalculation is generally carried out by most communication networks andthus can be utilised by embodiments of the present invention.

Preferably, the first station is a base transceiver station in acellular network. The second station is preferably a mobile station insaid cellular network. Preferably, the signals are burst, and the phaseis altered on a burst-by-burst basis.

As will be appreciated, aspects of the first invention can be used withaspects of the second invention and vice-versa.

According to a third aspect of the present invention, there is provideda first station for directional radio communication with the secondstation, said apparatus comprising transmitter means for transmitting asignal in a plurality of beam directions, each of said beam directionsbeing selectable; selection means for selecting a plurality of beamdirections in which a signal is to be transmitted from the first stationto the second station; and control means for controlling saidtransmitter means, wherein said control means is arranged toindividually control the power level of the signal transmitted in eachof said selected beam directions.

Preferably, said station is arranged to transmit first and secondsignals to said second station, said first station comprising phasealtering means for altering the phase of the first and second signals tobe sent in at least one of the selected beam directions; and whereinsaid control means is arranged to control the transmitter means totransmit the first and second signals in said plurality of beamdirections, whereby the phase of the first signal in at least two of thebeam directions differs, the phase of the second signal in said at leasttwo of the beam directions differs and the phase difference of saidfirst signal in said at least two beam directions is different from thephase difference of the second signal in said at least two beamdirections.

Preferably, the means for altering the phase comprises a phasemodulator. The phase modulator may be arranged between the input of abeam former of said transmitting means and a signal processor of thefirst station.

According to a fourth aspect of the present invention, there is provideda first station for directional radio communication with a second mobilestation, said apparatus comprising receiving means for receiving aplurality of consecutive signals transmitted by said second station,said signals being receivable from a plurality of different beamdirections; determining means for determining for at least one of thebeam directions the value of a parameter of at least one of signalsreceived from the second station in said at least one beam direction; alook-up table for providing power values corresponding to the determinedvalue; and transmitting means for transmitting a second signal to thesecond station in said at least one beam direction, the power level ofthe signal being determined by the power value looked-up in the look-uptable.

The determining means may be arranged to determine a mean value of theparameter for a plurality of signals. The parameter may be the energy ofthe signal.

Preferably, the transmitter means of either the third or the fourthaspect comprise an antenna array which is arranged to provide aplurality of signal beams in a plurality of different directions.

For a better understanding of the present invention and as to how thesame may be carried into effect, reference will now be made by way ofexample to the accompanying drawings in which:

FIG. 1 shows a schematic view of a base transceiver station (BTS) andits associated cell sectors;

FIG. 2 shows a simplified representation of an antenna array and thebase transceiver station;

FIG. 3 shows the fixed beam pattern provided by the antenna array ofFIG. 2;

FIG. 4 shows a schematic view of the digital signal processor of FIG. 2;

FIG. 5 illustrates the channel impulse response for four channels, outof the eight channels;

FIG. 6 illustrates the array pattern obtained when two adjacent beamsare selected and random phase difference is used;

FIG. 7 shows a similar pattern to that of FIG. 6 but shows the patterngenerated when the relative phase is varied from 100° to 130° in threedegree steps; and

FIGS. 8a-8 f show the simulated average array response when three beamsare selected, with various thresholds.

Reference will first be made to FIG. 1 in which three cell sectors 2defining a cell 3 of a cellular mobile telephone network are shown. Thethree cell sectors 2 are served by respective base transceiver stations(BTS) 4. Three separate base transceiver stations 4 are provided at thesame location. Each BTS 4 has a separate transceiver which transmits andreceives signals to and from a respective one of the three cell sectors2. Thus, one dedicated base transceiver station is provided for eachcell sector 2. The BTS 4 is thus able to communicate with mobilestations (MS) such as mobile telephones which are located in therespective cell sector 2.

The present embodiment is described in the context of a GSM (GlobalSystem for Mobile Communications) network. In the GSM system, afrequency/time division multiple access F/TDMA system is used. Data istransmitted between the BTS 4 and the MS in bursts. The data burstsinclude a training sequence which is a known sequence of data. Thepurpose of the training sequence will be described hereinafter. Eachdata burst is transmitted in a given frequency band in a predeterminedtime slot in that frequency band. The use of a directional antenna arrayallows space division multiple access also to be achieved. Thus, inembodiments of the present invention, each data burst will betransmitted in a given frequency band, in a given time slot, and in agiven direction. An associated channel can be defined for a given databurst transmitted in the given frequency, in the given time slot, and inthe given direction. As will be discussed in more detail hereinafter, insome embodiments of the present invention, the same data burst istransmitted in the same frequency band, in the same time slot but in twodifferent directions.

FIG. 2 shows a schematic view of one antenna array 6 of one BTS 4 whichacts as a transceiver; It should be appreciated that the array 6 shownin FIG. 2 only serves one of the three cell sectors 2 shown in FIG. 1.Another two antenna arrays 6 are provided to serve the other two cellsectors 2. The antenna array 6 has eight antenna elements a₁ . . . a₈.The elements a₁ . . . a₈ are arranged to have a spacing of a halfwavelength between each antenna element a₁ . . . a₈ and are arranged ina horizontal row in a straight line. Each antenna element a₁ . . . a₈ isarranged to transmit and receive signals and can have any suitableconstruction. Each antenna element a₁ . . . a₈ may be a dipole antenna,a patch antenna or any other suitable antenna. The eight antennaelements a₁ . . . a₈ together define a phased array antenna 6.

As is known, each antenna element a₁ . . . a₈ of the phased arrayantenna 6 is supplied with the same signal to be transmitted to a mobilestation MS. However, the phases of the signals supplied to therespective antenna elements a₁ . . . a₈ are shifted with respect to eachother. The differences in the phase relationship between the signalssupplied to the respective antenna elements a₁ . . . a₈ gives rise to adirectional radiation pattern. Thus, a signal from the BTS 4 can only betransmitted in certain directions in the cell sector 2 associated withthe array 6. The directional radiation pattern achieved by the array 6is a consequence of constructive and destructive interference whicharises between the signals which are phase shifted with respect to eachother and transmitted by each antenna element a₁ . . . a₈. In thisregard, reference is made to FIG. 3 which illustrates schematically thedirectional radiation pattern which is achieved with the antenna array6. It should be appreciated that in practice adjacent beams will overlapone another to ensure that all of the cell sector 2 is served by theantenna array 6. The antenna array 6 can be controlled to provide a beamb₁ . . . b₈ in any one of the eight directions illustrated in FIG. 3.For example, the antenna array 6 could be controlled to transmit asignal to a MS only in the direction of beam b₅ or only in the directionof beam b₆. As will be discussed in further detail hereinafter, it ispossible also to control the antenna array 6 to transmit a signal inmore than one beam direction at the same time. For example, a signal maybe transmitted in the two directions defined by beam b₅ and beam b₆.FIG. 3 is only a schematic representation of the eight possible beamdirections which can be achieved with the antenna array 6.

The relative phase of the signal provided at each antenna element a₁ . .. a₈ is controlled by Butler matrix circuitry 8 so that a signal can betransmitted in the desired beam direction or directions. The Butlermatrix circuitry 8 thus provides a phase shifting function. The Butlermatrix circuitry 8 has eight inputs 10 a-h from the BTS 4 and eightoutputs, one to each antenna element a₁ . . . a₈. The signals receivedby the respective inputs 10 a-h comprise the data bursts to betransmitted. Each of the eight inputs 10 a-h represents the beamdirection in which a given data burst could be transmitted. For example,when the Butler matrix circuitry 8 receives a signal on the first input10 a, the Butler matrix circuitry 8 applies the signal provided on input10 a to each of the antenna elements a₁ . . . a₈ with the required phasedifferences to cause beam b₁ to be produced so that the data burst istransmitted in the direction of beam b₁. Likewise, a signal provided oninput 10 b causes a beam in the direction of beam b₂ to be produced andso on.

As already discussed, the antenna elements a₁ . . . a₈ of the antennaarray 6 receive signals from a MS as well as transmit signals to a MS. Asignal transmitted by a MS will generally be received by each of theeight antenna elements a₁ . . . a₈. However, there will be a phasedifference between each of the signals received by the respectiveantenna elements a₁ . . . a₈. With the assistance of the Butler matrixcircuitry 8 it is possible to determine from the relative phases of thesignals received by the respective antenna elements a₁ . . . a₈ the beamdirection from which the signal has been received. The Butler matrixcircuitry 8 thus has eight inputs, one from each of the antenna elementsa₁ . . . a₈ for the signal received by each antenna element. The Butlermatrix circuitry 8 also has eight outputs 14 a-h. Each of the outputs 14a to 14 h corresponds to a particular beam direction from which a givendata burst could be received. For example, if the antenna array 6receives a signal from a MS from the direction of beam b₁, then theButler matrix circuitry 8 will output the received signal on output 14a. A received signal from the direction of beam b₂ will cause thereceived signal to be output from the Butler matrix circuitry 8 onoutput 14 b and so on. In practice, a signal is unlikely to be receivedfrom just a single beam direction due to side lobes and/or multipatheffects. However, the level or amplitude of the signal received in anumber of beam directions will often be quite low and as such can, insome embodiments of the present invention be disregarded. In summary,the Butler matrix circuitry 8 will receive on the antenna elements a₁ .. . a₈ eight versions of the same signal which are phase shifted withrespect to one another. From the relative phase shifts, it is possiblewith the assistance of the Butler matrix circuitry 8 to determine thedirection from which the received signal has been received and outputs asignal on a given output 14 a-h in dependence on the direction fromwhich the signal has been received.

It should be appreciated that in some environments, a single signal ordata burst from a MS may appear to come from more than one beamdirection due to reflection of the signal whilst it travels between theMS and the BTS 4, provided that the reflections have a relatively wideangular spread. The Butler matrix circuitry 8 will provide a signal oneach output 14 a-h corresponding to each of the beam directions fromwhich a given signal or data burst appears to come. Thus, the same databurst may be provided on more than one output 14 a-h of the Butlermatrix circuitry 8. However, the signals on the respective outputs 14a-h may be time delayed with respect to each other.

Each output 14 a-h of the Butler matrix circuitry 8 is connected to theinput of a respective amplifier 16 which amplifies the received signal.One amplifier 16 is provided for each output 14 a-h of the Butler matrixcircuitry 8. The amplified signal is then processed by a respectiveprocessor 18 which manipulates the amplified signal to reduce thefrequency of the received signal to the baseband frequency so that thesignal can be processed by the BTS 4. To achieve this, the processor 18removes the carrier frequency component from the input signal. Again,one processor 18 is provided for each output 14 a-h of the Butler matrixcircuitry 8. The received signal, which is in analogue form, is thenconverted into a digital signal by an analogue to digital (A/D)converter 20. Eight A/D converters 20 are provided, one for each output14 a-h of the Butler matrix circuitry 8. The digital signal is thereinput to a digital signal processor 21 via a respective input 19 a-h forfurther processing.

The digital signal processor 21 also has eight outputs 22 a-h, each ofwhich outputs a digital signal which represents the signal which is tobe transmitted to a given MS. The output 22 a-h selected represents thebeam direction in which the signal is to be transmitted. That digitalsignal is converted to an analogue signal by a digital to analogue (D/A)converter 23. One digital to analogue converter 23 is provided for eachoutput 22 a-h of the digital signal processor 21. The analogue signal isthen processed by processor 24 which is a modulator which modulates ontothe carrier frequency the analogue signal to be transmitted. Prior tothe processing of the signal by the processor 24, the signal is at thebaseband frequency. The resulting signal is then output to a respectivephase modulator 25. One phase modulator 25 is provided for eachprocessor 24 and the output of the respective phase modulators 25 arepassed to respective power amplifiers 26 which amplify the respectivesignals. Again one power amplifier is provided for each phase Modulator25. The output of the respective power amplifiers 26 are provided torespective inputs 10 a-h of the Butler matrix circuitry 8. Thus, aprocessor 24, an amplifier 26 and a phase modulator 25 are provided foreach output 22 a-h of the digital signal processor 21.

Each phase modulator 25 is arranged to change the phase of therespective signal to be applied to the corresponding input 10 a-h of theButler matrix circuitry 8, on a burst by burst basis. In other words thephase of the signal applied to a given input 10 a-h of the Butler matrixcircuitry 8 differs for consecutive bursts. In the preferred embodimentof the present invention, the phase modulators 25 randomly alter thephase of the signals on a burst by burst basis. However, in onemodification the phase is not altered randomly but is instead altered ina predetermined manner. The function of the phase modulators 25 will bedescribed in more detail hereinafter. As mentioned hereinbefore, one ormore of the inputs 10 a-h of the Butler matrix circuitry may be arrangedto have the same signal applied thereto. In preferred embodiments, thephase of the signal applied for a given burst to different inputs willbe different at different inputs. This will be described in more. detailhereinafter.

Reference will now be made to FIG. 4 which schematically illustrates inmore detail the digital signal processor 21. It should be appreciatedthat the various blocks illustrated in FIG. 4 do not necessarilycorrespond to separate elements of An actual digital signal processor 21embodying the present invention. In particular, the various blocksillustrated in FIG. 4 correspond to various functions carried out by thedigital signal processor 21. In one embodiment of the present invention,the digital signal processor 21 is at least partially implemented inintegrated circuitry and several functions may be carried out by thesame element.

Each signal received by the digital signal processor 21 on therespective inputs 19 a-h is input to a respective channel impulseresponse (CIR) estimator block 30. The CIR estimator block 30 includesmemory capacity in which the estimated channel impulse response isstored. The CIR estimator block 30 also includes memory capacity fortemporarily storing a portion of the received signal. The channelimpulse response estimator block 30 is arranged to estimate the channelimpulse response of the channel of the respective input 19 a-h. Asalready discussed an associated channel can be defined for the givendata burst transmitted in the selected frequency band, the allocatedtime slot and the beam direction from which the signal is received. Thebeam direction from which a signal is received is ascertained with thehelp of the Butler matrix circuitry 8 so that a signal received at input19 a of the digital signal processor represents mainly the signal thathas been received from the direction of beam b₁ and so on. It should beappreciated that the signal received at a given input may also includethe side lobes of the signal received on, for example, adjacent inputs.

Each data burst which is transmitted from a mobile station MS to the BTS4 includes a training sequence TS. However, the training sequenceTS_(RX) which is received by the BTS 4 is affected due to noise and alsodue to multipath effects which leads to interference between adjacentbits of the training sequence. TS_(RX) is also affected by interferencefrom other mobile stations, for example mobile stations located in othercells or cell sectors using the same frequency which may causeco-channel interference. As will be appreciated, a given signal from theMS may follow more than one path to reach the BTS and more than oneversion of the given signal may be detected by the antenna array 6 froma given direction. The training sequence TS_(RX) which is received frominput 19 a is cross correlated by the CIR estimator block 30 with areference training sequence TS_(REF) stored in a data store 32. Thereference training sequence TS_(REF) is the same as the trainingsequence which is initially transmitted by the mobile station. Inpractice the received training sequence TS_(RX) is a signal modulatedonto a carrier frequency while the reference training sequence TS_(REF)is stored as a bit sequence in the data store 32. Accordingly, beforethe cross-correlation is carried out, the stored reference trainingsequence is similarly modulated. In other words the distorted trainingsequence received by the BTS 4 is correlated with the undistortedversion of the training sequence. In an alternative embodiment of theinvention, the received training sequence is demodulated prior to itscorrelation with the reference training sequence. In this case, thereference training sequence would again have the same form as thereceived training sequence. In other words, the reference trainingsequence is not modulated.

The reference training sequence TS_(REF) and the received trainingsequence TS_(RX) each are of length L corresponding to L bits of dataand may for example be 26 bits. The exact location of the receivedtraining sequence TS_(RX) within the allotted time slot may beuncertain. This is because the distance of the mobile station MS fromthe BTS 4 will influence the position of the data burst sent by the MSwithin the allotted time slot. For example, if a mobile station MS isrelatively far from the BTS 4, the training sequence may occur later inthe allotted time slot as compared to the situation where the mobilestation MS is close to the BTS 4.

To take into account the uncertainty of the position of the receivedtraining sequence TS_(RX) within the allotted time slot, the receivedtraining sequence TS_(RX) is correlated with the reference trainingsequence TS_(REF) n times. Typically, n may be for example 7 or 9. It ispreferred that n be an odd number. The n correlations will typically beon either side of the maximum obtained correlation. The relativeposition of the received training sequence TS_(RX) with respect to thereference training sequence TS_(REF) is shifted by one position betweeneach successive correlation. Each position is equivalent to one bit inthe training sequence and represents one delay segment. Each singlecorrelation of the received training sequence TS_(RX) with the referencetraining sequence TS_(REF) gives rise to a tap which is representativeof the channel impulse response for that correlation. The n separatecorrelations gives rise to a tap sequence having n values. It should beappreciated that some of the taps may be zero or very small. Thistypically will occur at one or other or both ends of the tap sequence,the maximum value typically being in the middle region of the tapsequence.

Reference is now made to FIG. 5 which shows the channel impulse responsefor four of the eight possible channels corresponding to the eightspatial directions. In other words, FIG. 5 shows the channel impulseresponse for four channels corresponding to a given data burst receivedin four of the eight beam directions from the mobile station, the databurst being in a given frequency band and in a given time slot. The xaxis of each of the graphs is a measure of time delay whilst the y axisis a measure of relative power. Each of the lines (or taps) marked onthe graph represents the multipath signal received corresponding to agiven correlation delay. Each graph will have n lines or taps, with onetap corresponding to each correlation.

From the estimated channel impulse response, it is possible to determinethe location of the training sequence within the allotted time slot. Thelargest tap values will be obtained when the best correlation betweenthe received training sequence TS_(RX) and the reference trainingsequence TS_(REF) is achieved.

The CIR estimator block 30 also determines for each channel the five (orany other suitable number) consecutive taps which give the maximumenergy. The maximum energy for a given channel is calculated as follows:$\begin{matrix}{E = {\sum\limits_{j = 1}^{5}\quad \left( h_{j} \right)^{2}}} & (I)\end{matrix}$

where h represents the tap amplitude resulting from a cross correlationof the reference training sequence TS_(REF) with the received trainingsequence TS_(RX). The CIR estimator block 30 estimates the maximumenergy for a given channel by using a sliding window technique. In otherwords, the CIR estimator block 30 considers each of five adjacent valuesand calculates the energy from those five values. The five adjacentvalues giving the maximum energy are selected as representative of theimpulse response of that channel.

The energy can be regarded as being a measure of the strength of thedesired signal from a given MS received by the BTS 4 from a givendirection. This process is carried out for each of the eight channelswhich represent the eight different directions from which the same databurst could be received. The signal which is received with the maximumenergy has followed a path which provides the minimum attenuation ofthat signal.

A respective analysis block 34 is connected to each CIR estimator block30. Each analysis block 34 is arranged to store the maximum energy valuecalculated, by the CIR estimator block 30 to which the respectiveanalysis block 34 is connected for the respective channel, from the fiveadjacent values selected by the given CIR estimator block 30 as beingrepresentative of the channel impulse response. The analysis block 34 isalso arranged to store the calculated maximum energy for the N-1preceding data bursts as well as the energy for the current data burst.The analysis block 34 is arranged to calculate the average energy{overscore (E)}_(i) for a particularly channel over N bursts using thefollowing equation: $\begin{matrix}{{\overset{\_}{E}}_{i} = {\frac{1}{N} \cdot {\sum\limits_{K = 1}^{N}\quad E_{k}}}} & ({II})\end{matrix}$

where: N=the number of bursts over which the average is computed;

i is the beam number; and

E_(k) is the maximum energy calculated using equation (I) for the ithdata burst.

N can have any suitable value in practice and may be in the range of1-100. However, it should be appreciated that in some embodiments of thepresent invention N can be greater than 100.

The mean energy of the ith beam is on average, a measure of the aptitudeof the ith channel to transport desired information in the up-linkdirection. Although the channels in the up-link and down-link are notstatistically the same, {overscore (E)}_(i) can be regarded as giving,on average, some useful information for the down-link transmission.

Thus, a value for {overscore (E)}_(i) which exceeds a given threshold isan indication that during the N last received bursts the attenuation ofthe ith channel was on average lower than a typical channel attenuationand that therefore the power for down-link transmission of the nextburst can be reduced and vice versa. It should be appreciated that asthis control operates by averaging energies of the last N receivedbursts, the system is effective on average and not necessarily effectivein every burst.

The calculated average energy values for the respective beams are outputby the respective analysis blocks 34 to a look-up table block 101. Thelook-up table block 101 first quantises the average energy valuesreceived from each of the eight beam directions. The look-up table block101 has a look-up table with the power level associated with each of thepossible quantised energy values. This power level is the power level tobe used in a particular beam direction for transmitting a signal to agiven MS. It should be appreciated that the look-up table may includethe quantised energy levels with the associated power level. The look-uptable may thus be addressed by the calculated average energy values.Thus, the input to the look-up table is the calculated average energyand the output is the output power for the corresponding output poweramplifier 26 or the associated control signal to set the desired power.

The look-up table is generated such that a reliable relationship betweenthe average received energy and the output power can be established. Theequivalent average path loss of each channel and hence the power levelfor the down-link transmission can be estimated based on the knowledgeof the output transmission power employed by the mobile station and theestimate of the mean received energy from the given mobile station. Inthis way values for the look-up table can be generated. The power levelvalues are estimated so that down-link transmission power in a givenbeam direction achieves on average a certain level of signal in thedesired mobile station MS. The contents of the look-up table may befixed or alternatively may be dynamically changed, for example, inaccordance with power measuring reports received from the mobile stationMS in question. Additional information can be used to compute thelook-up table. For example the distance between the mobile station MSand the BTS may be taken into account. This distance can be calculatedfrom timing advance information. The validity of known attenuation lawsin the radio channel can also be taken into account. Different radioenvironments may have different attenuation laws. By determining whichattenuation law is applicable, the conditions of a particular radioenvironment can be taken into consideration.

It should be noted that as the correlation between the up-link anddown-link channels increases the channels become more and morereciprocal and the amount of previous information required to make adecision on the power level can be reduced. In other words, N can bereduced. The degree of correlation between the up-link and down-linkchannels can vary in dependence on the location and nature of the cellor cell sector. It should also be appreciated that the degree ofcorrelation of a given location cell or cell sector may vary over time.The value of N may therefore be different for different base transceiverstations. Additionally, the radio environment of a cell may vary withtime and/or location of the mobile station within the cell. In thelimit, when up-link and down-link channels are fully reciprocal, N=1 andin this situation the level of power to be used during down-linktransmission can be estimated directly from the amount of energyreceived in the previous up-link burst. However this limit situationdoes not usually occur. It should be appreciated that in embodiments ofthe present invention, N can be fixed or can be variable. In the lattercase, N can be varied in accordance with certain parameters such as, forexample, signal quality reports from the mobile station MS in questionso that maximum performance can be achieved.

In one modification to the present invention, instead of calculating theaverage energy values for the respective beams and using the look-uptable to make an association between those average values and the powerlevel, the look-up table can make an association between the distancebetween the base station and the mobile station and the power level.Alternatively, an association can be made between the type of radioenvironment and the power level. Instead of using average values,instantaneous values might be used and in particular the instantaneousenergy values. However, it is preferred that the average energy be usedand the association be between the average energy and the power level bemade. In all of these described alternatives, a look-up table wouldagain be provided to make the necessary association.

Each analysis block 34 may also analyse the channel impulse responsesdetermined by the CIR block 30 to ascertain the minimum delay τ. Thedelay is a measure of the position of the received training sequenceTS_(RX) in the allotted time slot and hence is a relative measure of thedistance travelled by a signal between the mobile station and the BTS 4.The channel with the minimum delay has the signal which has travelledthe shortest distance. This shortest distance may in certain casesrepresent the line of sight path between the mobile station MS and theBTS 4.

The analysis block 34 may be arranged to determine the position of thebeginning of the window defining the five values providing the maximumenergy. The time delay is then determined based on the time between areference point and the beginning of the window. That reference pointmay be the common time when all received training sequences in eachbranch start to be correlated, the time corresponding to the earliestwindow edge of all the branches or an equivalent common point. In orderto accurately compare the various delays of the different channels, acommon timing scale is adopted which relies on the synchronisationsignal provided by the BTS 4 in order to control the TDMA mode ofoperation. In other words, the position of the received trainingsequence TS_(RX) in the allotted time slot is a measure of the timedelay. It should be appreciated that in known GSM systems, the delay fora given channel is calculated in order to provide timing advanceinformation. Timing advance information is used to ensure that a signaltransmitted by the mobile station to the BTS falls within its allottedtime slot. The timing advance information can be determined based on thecalculated relative delay and the current timing advance information. Ifthe mobile station MS is far from the base station, then the MS will beinstructed by the BTS to send its data burst earlier than if the mobilestation MS is close to the BTS. The results of this analysis may beinput to the look-up table block 101.

The look-up table block 101 may also make a determination as to whichbeams are actually to be selected as well as determining the power ofthe selected beam or beams. There are a number of different ways inwhich this can be achieved. If, for example, the look-up table block 101is to determine a single beam direction for a given burst, then thelook-up table block 101 may ascertain which channel and hence which beamdirection has the desired maximum energy for a given data burst in agiven frequency band in a given time slot. This means that the beamdirection from which the strongest version of the given data burst isreceived can be ascertained. This direction may be used as the selectedbeam direction. The power of the beam would be that which is determinedfrom the look-up table of the look-up table block 101 for the determinedaverage energy for the respective beam direction. Alternatively, thelook-up table block 101 may ascertain which of the channels has aminimum delay. In other words, the channel and hence the beam directionhaving the data burst which has followed the shortest path can beascertained and used as the selected beam direction for a given databurst. The power would again be determined by the look-up table of thelook-up table block 101 using the calculated average energy.

In preferred embodiments of the present invention, more than one beamdirection can be selected by the look-up table block 101 for a givendata burst. For example, the two directions from which the strongestversion of a given data signal are received can be selected as the givenbeam directions. Likewise, the two beam directions providing the signalwith the least delay may be selected as the beam directions. It would ofcourse be possible for the look-up table block 100 to ascertain thedirection from which the strongest signal is received as well as thedirection having the least delay and selecting those two directions asthe selected directions. In these embodiments the power of at least onebut preferably all of the selected beams is set in accordance withvalues ascertained from the look-up table of the look-up table block101. In one embodiment of the invention, three beams are selected. In afurther alternative, one beam direction could be selected as outlinedhereinbefore and the further beam or beams selected could be directlyadjacent the first selected beam direction. In all of these variations,it is preferred that the power level be obtained using the look-uptable, as outlined hereinbefore. By controlling the power of the beamsindividually, the amount of interference which is generated can bereduced.

The look-up table block 101 provides an output to generating block 38which indicates which beam directions are to be used to transmit signalsfrom the BTS 4 to the MS and also the appropriate power level to be usedwith each of those beam directions.

Generating block 38 is responsible for generating the signals which areto be output from the digital signal processor 21. The generating block38 has an input 40 representative of the speech and/or information to betransmitted to the mobile station MS. Generating block 38 is responsiblefor encoding the speech or information to be sent to the mobile stationMS and includes a training sequence and a synchronising sequence withinthe signals. Generating block 38 is also responsible for production ofthe modulating signals. Based on the generated signal and determinedbeam direction, generating block 38 provides signals on the respectiveoutputs 22 a-h of the digital signal processor 21. The generating block38 also provides an output 50 which is used to control the amplificationprovided by the respective amplifiers 26 to ensure that the signalstransmitted in the one or more beam directions have the required powerlevels. This output 50 comprises the power level or control signal forthe power level determined from the look-up table of the look-up tableblock 101. It should be appreciated that the power level for each of theamplifiers 26 can be individually set.

The output of the channel impulse response block 30 is also used toequalise and match the signals received from the mobile station MS. Inparticular, the effects of intersymbol interference resulting frommultipath propagation can be removed or alleviated from the receivedsignal by the matched filter (MF) and equaliser block 42. It should beappreciated that the matched filter (MF) and equalizer block 42 has aninput (not shown) to receive the received signal from the MS. The outputof each block 42 is received by recovery block 44 which is responsiblefor recovering the speech and/or the information sent by the MS. Thesteps carried out by the recovery block include demodulating anddecoding the signal. The recovered speech or information is output onoutput 46.

Reference will now be made to FIGS. 6 to 9 in order to explain thepurpose of the modulators 25. The situation where no phase modulators 25are provided will first be considered. If more than one beam is selectedand the beams are adjacent, then the adjacent beams will tend tointeract in the region of overlap between the beams. The degree ofinteraction in the region of overlap is largely determined by the phasedifference between signals applied to the inputs 10 of the Butler matrixcircuitry 8 for the respective beam directions which have been selected.

In theory, the phase of the signals provided at the inputs 10 of theButler matrix circuitry 8 should be the same. However, each signalarrives at the respective input 10 of the Butler matrix circuitry 8 viaits own power amplifier 26 and cable 27. Although the power amplifiers26 and cables 27 are similar, they are not identical. This means thatthe phase shift caused by these elements is not identical. Thusidentical signals output on adjacent outputs 22 of the digital signalprocessor 21 at the same time will arrive at the respective input 10 ofthe Butler matrix circuitry 8 with different phases. The phases of thesignals are thus different and the phase difference between the signalsis unknown. This leads to uncertainty in the resulting beam patterndefined by the selected beams particularly in the region of overlap. Thedifference in the phases of two adjacent beams could cause there to be anull area in this region of overlap. This would mean that if a mobilestation MS were in the null region, the mobile station MS would not beserved by the base station BTS.

To deal with this problem it is proposed in embodiments of the presentinvention to provide a phase modulator 25 in each path from the digitalsignal processor 21 to the Butler matrix circuitry 8. The phasemodulators 25 are arranged to randomly alter the phase of each signalwhich passes therethrough. The phase shifts are changed on aburst-by-burst basis. By randomly changing the phase of the signalsapplied to the respective inputs 10 of the Butler matrix circuitry, asmooth pattern will be obtained on average with no null regions betweenthe two adjacent beams. In this regard, reference is made to FIG. 6which shows the beam pattern obtained when the phase modulators areused. In the illustrated examples beams b₅ and b₆ have been selected. Inparticular FIG. 6 illustrates the different beam pattern responses whichare obtained when the phases of the signals of beams b₅ and b₆ arerandomly altered by the modulators 25. As can be seen from this figure,there is on average no null regions between the two beams.

The phase difference between beams b₅ and b₆ varies between −Π and +Π.The pattern illustrated in FIG. 6 shows the pattern obtained for 100bursts.

Reference is now made to FIG. 7 which shows a similar pattern to thebeam pattern of FIG. 6 but shows the beam pattern generated when therelative phase (i.e. phase difference) of beams b₅ and b₆ is varied from100° to 130° in 3° steps. When the relative phase is 100°, it can beseen that there is a null region 120 between the beams b₅ and b₆.However, where the relative phase increases, for example to about 120°,a generally smooth response is obtained covering the whole of theangular sector for beams b₅ and b₆. Finally, a peaked response isobtained when the relative phase between beams b₁ and b₆ is 130°. Byrandomly varying the relative phase using the phase modulators 25, itcan be ensured that the null region created, for example when therelative phase between the beams is 100° would only last for a singleburst as the phase of the signals are randomly changed on a burst byburst basis.

Reference will now be made to FIG. 8, which illustrates the resultingpattern when three beams are selected. In relation to FIG. 8, it isassumed that three adjacent beams are selected, for example beams b₅, b₆and b₇. The phase modulators 25 change the phase of the three signals tobe input to the Butler matrix circuitry 8 on inputs 10 e, 10 f and 10 hon a burst by burst basis creating a moderate spatial modulation of theresulting beam pattern obtained where more than one beam is selected.The power for the beams can be selected as hereinbefore described or canbe selected in any other way. It is assumed in the following that theamplitude of the signal x(t) to be transmitted is a for beam b5, 1 forbeam b6 and a for beam b7 where a ≦1.

In simulations the probability that the array response, that is theresulting pattern formed where more than one beam is selected, exceeds agiven threshold was determined. The probability was computed for anangular sector corresponding to beams b₅, b₆ and b₇. The graphs shown inFIG. 8 represent a measure of the average response of the array for arelatively large number of bursts. The thresholds used representparticular percentages of the maximum response achieved when a singlebeam is selected. FIGS. 8a to 8 d show the results of the simulationswhere the upper thresholds are 80%, 70%, 50% and 30% respectively. Theparameter a was varied from 0.1 to 1 in steps of 0.1. The probabilitythat the array response exceeds the selected threshold is represented bythe y axis whilst the angle in degrees corresponding to the three beamsb₅, b₆ and b₇ is represented by the x axis. These graphs can beconsidered to show an average array response since they represent thestatistics of a large number of successive bursts.

Consider FIG. 8b which has an upper threshold of 70%. This means thatthe signals which exceed a threshold of 70% of the maximum signalobtained with just one beam are considered. For a=0, the response isonly due to the central beam, b₆ and it can be seen from FIG. 8b thatthe response is 100% between approximately 107° to 117° and 0% in therest of the angular spread. As the value of a is increased, the signalson each side of the central response become more important. In otherwords the contribution from signals b₅ and b₇ starts to become moreimportant. As a approaches 1, there are already “peaks” of signals onboth sides of the central beam. In other words, as a approaches 1, theamplitude and power of beams b₅ and b₇ approach that of b₆ and theprobability figures in the corresponding angular sectors are increasedto levels similar to those obtained in the angular sector for beam b₆.Beam b₅ covers the angular sector from 90° to 105°, beam b₆ from 105° to120° and beam b₇ from 120° to 135°.

As can be seen from any of FIGS. 8a to 8 d, the average response (orprobability that the stated threshold is exceeded) changes with a. Witha=0, a response with a central “rectangle” is obtained. As a increases,the signals start to exceed the threshold on both sides of therectangle.

FIGS. 8e and 8 f show graphs similar to those of FIGS. 8a to 8 d butinstead illustrate the probabilities of not exceeding a given threshold.In particular FIG. 8e represents the probability of not exceeding athreshold of 10% whilst FIG. 8f represents the probability of notexceeding a threshold of 20%. As with the case of an upper threshold,there is a clear effect of the beam signal amplitude on probabilities.The lower the threshold is set, the lower the probability that a signalwill be below that threshold and vice versa. As the parameter a isincreased, the amplitude of the signal transmitted will have anincreased amplitude and the probability that the signal will not surpassthe threshold is reduced. The areas or angular sectors corresponding tothe beams which are required to be free of interference can becontrolled by using random phase modulation in association with beampower control.

It should be appreciated that each beam could have its own parameter,e.g. a for beam b₅, b for beam b₆ and c for beam b₇. In the specificexample described above, a=c and b=1. However, a, b and c could all bedifferent. a, b and c may be determined based on the results obtainedfrom the look-up table block 101 for the power level. However, in otherembodiments of the present invention, other ways can be used tocalculate the relative amplitudes of the beams. For example, the valuesof a, b and c could be determined based on the amount of desired signalsreceived in the corresponding up-link beams etc. It should be pointedout that the power is closely related to amplitude. In particularpower=(amplitude)².

From the results illustrated in FIG. 8, it can be seen that where theamplitude of the three selected beams can be freely chosen, the shape ofthe probability plot can be modified quite flexibly. Since, on average,the envelope of the signal will follow the probability plot it ispossible to achieve a dynamic control over the shape of the arrayresponse. The phase modulators 25 allow this control over the shape ofthe array response to be achieved.

In the above described embodiment, the phase modulators 25 are arrangedto randomly alter the phase of the signals passing therethrough. Howeverin alternative embodiments, a non-random pattern may be used to alterthe phase of the signals. The predetermined pattern can be modified totake into account one or more of the following factors: radioenvironment signal levels, distance between mobile station and BTS, andthe like. It is preferred that the phase of each signal be altered on aburst-by-burst basis. In this described modification it is preferredthat a calibration system for computing the desired phase values beprovided.

In the embodiment described hereinbefore, the phases of successivesignals applied to a given input are varied randomly as are the phasesof successive signals applied to adjacent inputs. In one modification tothis embodiment, the phase of the signals applied to a given output maynot be varied, whereas the phase of the signals applied to an adjacentinput are varied on a burst by burst basis.

It should be appreciated that whilst the above described embodiment hasbeen implemented in a GSM cellular communication network, it is possiblethat the present invention can be used with other digital cellularcommunication networks as well as analogue cellular networks. The abovedescribed embodiment uses a phased array having eight elements. It is ofcourse possible or the array to have any number of elements.Alternatively, the phased array could be replaced by discretedirectional antennae each of which radiates a beam in a given direction.The Butler matrix circuitry can be replaced by any other suitable phaseshifting circuitry, where such circuitry is required. The Butler matrixcircuitry is an analogue beam former. It is of course possible to use adigital beam former DBF or any other suitable type of analogue beamformer. The array may be controlled to produce more than eight beams,even if only eight elements are provided, depending on the signalssupplied to those elements.

The number and/or direction of beams which are used to transmit signalsto a mobile station may be different to or the same as the number and/ordirection of the beams received by the BTS from the MS.

The power level of the beams may be modified after a level has beenselected by the look-up table block in order to take in account externalfactors such as the quality of the received signals etc.

It is also possible for a plurality of phased arrays to be provided. Thephased arrays may provide a different number of beams. When a wideangular spread is required, the array having the lower number ofelements is used and when a relatively narrow beam is required, thearray having the larger number of elements is used.

As will be appreciated, the above embodiment has been described asproviding eight outputs from the Butler matrix circuitry. It should beappreciated that in practice a number of different channels will beoutput on each output of the Butler matrix at the same time. Thosechannels may be different frequency bands. The channels for differenttime slots will also be provided on the respective outputs. Whilstindividual amplifiers, processors, phase modulators, analogue to digitalconverters and digital to analogue converters have been shown, these inpractice may be each provided by a single element which has a pluralityof inputs and outputs.

It should be appreciated that embodiments of the present invention haveapplications other than just in cellular communication networks. Forexample, embodiments of the present invention may be used in anyenvironment which requires directional radio communication. For example,this technique may be used in Private Radio Networks or the like.

What is claimed is:
 1. A method of directional radio communicationbetween a first station and a second station, said method comprising thesteps of: defining at the first station a plurality of beam directionsfor transmitting signals to said second station, each of said beamdirections being selectable and being generated by an array of antennaelements, each of said elements receiving a signal to be transmitted;selecting a plurality of beam directions at said first station in whichsaid signal is to be transmitted from said first station to said secondstation; transmitting said signal in said plurality of beam directions,the power level of the signal transmitted in each of said selectedplurality of beam directions being individually selectable, whereinfirst and second sequential signals are to be transmitted to said secondstation, said transmitting step further comprising altering the phase ofthe first and second signals to be transmitted in at least one of saidselected beam directions defined by said antenna array; whereby thephase of the first signal transmitted in at least two of the beamdirections differs, the phase of the second signal transmitted in saidat least two beam directions differs and the phase difference of thefirst signal transmitted in said at least two beam directions isdifferent from the phase difference of said second signal transmitted insaid at least two beam directions.
 2. A method as claimed in claim 1,wherein said at least two beam directions are adjacent.
 3. A method asclaimed in claim 1, wherein said first station is arranged to transmit amultiplicity of consecutive signals to said second station and the phaseof each signal for at least one beam direction is altered such that thephase difference between each consecutive signal transmitted in theselected beam directions is different for consecutive signals.
 4. Amethod as claimed in claim 3, wherein the phase of said consecutivesignals is randomly altered.
 5. A method as claimed in claim 3, whereinthe phase of consecutive signals is altered in accordance with apredetermined pattern.
 6. A method as claimed in claim 1 comprising:receiving at said first station a plurality of consecutive signals fromsaid second station, said signals being receivable from a plurality ofbeam directions; determining for at least one of said beam directions avalue for a parameter for at least one signal received from said secondstation in said at least one beam direction; looking up in a look-uptable a power value corresponding to the determined value; andtransmitting a signal to the second station in said at least one beamdirection, the power of the signal transmitted in said at least one beamdirection being determined by the power value looked up in the look-uptable.
 7. A method as claimed in claim 6, wherein a mean value of theparameter for a plurality of signals is determined and a power valuecorresponding to the determined mean value is looked up in said look-uptable.
 8. A method as claimed in claim 7, wherein the parameter is theenergy of the signals.
 9. A method as claimed in claim 7, wherein saidmean energy determined in said determining step is quantized and thequantized mean energy is associated by the look-up table with acorresponding power value.
 10. A method as claimed in claim 6, whereinsaid parameter is one or more of the following parameters: instantaneousenergy of a signal; type of radio environment; or distance between thefirst and second stations.
 11. A method as claimed in claim 6, whereinsaid power value represents the power of the signal to be transmitted insaid beam direction.
 12. A method as claimed in claim 6, wherein saidpower value comprises a control value for controlling the setting of thepower level for a signal in a given beam direction.
 13. A method asclaimed in claim 6, wherein the values in the look-up table are alteredin accordance with a parameter of said first station and/or said secondstation.
 14. A method as claimed in claim 12, wherein the values of thelook-up table are altered in accordance with the power measuring reportsreceived from said second station.
 15. A method as claimed in claim 6,wherein the values for said look-up table are determined based on one ormore of the following: transmission power used by said second station;distance between the first and second station; the mean energy of thesignals received from the second station in a given beam direction;distance between the first and second station; the radio environment;the validity of a known attenuation law in a channel defined between thefirst and second stations.
 16. A method as claimed in claim 7, whereinthe number of signals used to calculate the mean value is varied.
 17. Amethod as claimed in claim 16, wherein the number of signals used tocalculate the mean value is dependent on signal quality reports from thesecond station.
 18. A method as claimed in claim 8, wherein the energyof each of said signals received in a given beam direction is determinedfrom the channel impulse response.
 19. A method as claimed in claim 6,wherein two beam directions are selected for transmission of a signal tosaid second station.
 20. A method as claimed in claim 6, wherein threebeam directions are selected for transmission of a signal to said secondstation.
 21. A method as claimed in claim 6, wherein said first stationis a base transceiver station in a cellular network.
 22. A method asclaimed in claim 6, wherein said second station is a mobile station in acellular network.
 23. A method as claimed in claim 21, wherein saidsignals are bursts and said phase is altered on a burst-by-burst basis.24. A first station for directional radio communication with a secondstation, said first station comprising: a transmitter comprising anantenna array having a plurality of antenna elements for transmitting asignal in a plurality of beam directions, said beam directions beingselectable, said signal being applied to each of said antenna elementsto provide said plurality of beam directions; circuitry for selecting aplurality of beam directions in which first and second signals are to betransmitted from the first station to the second station; circuitry foraltering the phase of the first and second signals to be sent in atleast one of the selected beam directions; and a controller forcontrolling said transmitter, wherein said controller is arranged toindividually control the power level of the signal transmitted in eachof said selected beam directions, and the controller being arranged tocontrol the transmitter to transmit the first and second signals in saidplurality of beam directions, whereby the phase of the first signal inat
 25. A first station as claimed in claim 24, wherein said circuitryfor altering the phase comprises a phase modulator.
 26. A first stationas claimed in claim 25, wherein said phase modulator is arranged betweenthe input of a beam former of said transmitter and a signal processor ofthe first station.
 27. A first station as claimed in claim 24, whereinthe transmitter means comprises an antenna array which is arranged toprovide a plurality of signal beams in a plurality of differentdirections.
 28. A first station as claimed in claim 24, wherein saidfirst station is a base transceiver station in a cellulartelecommunications network.